1. Field of the Invention
The present invention relates to a particle sensor for detecting the solid particles contained in a fluid.
2. Description of Related Art
When solid particles are present in a fluid (a liquid or a gas), it is necessary in some cases to detect the presence of the solid particles. Detection of such solid particles is particularly important when the solid particles present in the fluid badly affect the intended action of the fluid.
Lubricating oils (e.g. engine oils) are used in internal combustion engines of automobiles or heavy machinery in order to reduce the frictional resistance and abrasion of the rotating surfaces and sliding surfaces of the engines. Operation of such internal combustion engines generates solid particles (e.g. metal fine particles) caused by abrasion, and the solid particles are taken into the lubricating oils and accelerate, in some cases, the abrasion of the rotating surfaces and sliding surfaces. The solid particles, etc. present in the lubricating oils are ordinarily removed by the use of a filter such as oil filter or the like. The condition of the lubricating oils can be monitored in detail by detecting the solid particles present in the lubricating oils.
Hydraulic oils and flushing oils are used in power transmission mechanisms (e.g. transmission), oil hydraulic pipe systems (e.g. oil hydraulic servo valve), rolling, pressing, etc. It is important to control the condition of these oils by detecting the solid particles present in the oils and/or examining the viscosities of the oils.
It is also important to detect the particles suspended in the air or examine their concentration in the air in order to monitor the level of air pollution. Such detection includes detection of the particles suspended in the exhaust gas discharged from a plant, a factory or the like.
The applicant proposed, as a sensor for detecting solid particles contained in a fluid, an axial flow particle sensor comprising:
a sensor element comprising (a) a vibrating section having such a mass that is sensitive to the collision of solid particles contained in a fluid, with the vibrating section and (b) a detecting and converting section for detecting the vibration of the vibrating section caused by said collision and converting the vibration to electric signals, PA1 a housing for fixing said sensor element, PA1 a fluid inlet, and PA1 a fluid outlet, PA1 a sensor element comprising (a) a vibrating section having such a mass that is sensitive to the collision of solid particles contained in a fluid, with the vibrating section and (b) a detecting section for detecting the vibration of the vibrating section caused by said collision and converting the vibration to electric signals, PA1 a housing for fixing said sensor element, PA1 a fluid inlet, and PA1 a fluid outlet, PA1 a sensor element comprising (a) a vibrating section having such a mass that is sensitive to the collision of solid particles contained in a fluid, with the vibrating section and (b) a detecting section for detecting the vibration of the vibrating section caused by said collision and converting the vibration to electric signals, PA1 a housing for fixing said sensor element, PA1 a fluid inlet, and PA1 a fluid outlet, PA1 a sensor element comprising (a) a vibrating section having such a mass that is sensitive to the collision of solid particles contained in a fluid, with the vibrating section and (b) a detecting section for detecting the vibration of the vibrating section caused by said collision and converting the vibration to electric signals, PA1 a housing for fixing said sensor element, PA1 a fluid inlet, and PA1 a fluid outlet, PA1 a sensor element comprising (a) a vibrating section having such a mass that is sensitive to the collision of solid particles contained in a fluid, with the vibrating section and (b) a detecting section for detecting the vibration of the vibrating section caused by said collision and converting the vibration to electric signals, PA1 a housing for fixing said sensor element, PA1 a fluid inlet, and PA1 a fluid outlet, PA1 a sensor element comprising (a) a vibrating section having such a mass that is sensitive to the collision of solid particles contained in a fluid, with the vibrating section and (b) a detecting section for detecting the vibration of the vibrating section caused by said collision and converting the vibration to electric signals, PA1 a housing for fixing said sensor element, PA1 a fluid inlet, and PA1 a fluid outlet, PA1 a sensor element comprising (a) a vibrating section having such a mass that is sensitive to the collision of solid particles contained in a fluid, with the vibrating section and (b) a detecting section for detecting the vibration of the vibrating section caused by said collision and converting the vibration to electric signals, PA1 a housing for fixing said sensor element, PA1 a fluid inlet, and PA1 a fluid outlet,
in which axial flow particle sensor said fluid outlet is provided so that the sensor element is between the fluid inlet and the fluid outlet, throughholes are formed in the sensor element in at least part of the vicinity of the vibrating section, and the vibrating section of the sensor element is provided in the stream of the fluid formed by the fluid inlet so that the fluid from the fluid inlet can pass through the throughholes and leave the particle sensor from the fluid outlet (European Patent Publication 0769695-A2).
FIG. 5 shows an example of the structure of the above axial flow particle sensor. In this structure, the flow path of a fluid is constituted by a nozzle 33 forming the inlet 32 of the fluid, a nozzle 35 forming the outlet of the fluid, and a housing 30 to which the nozzles 33 and 35 are fixed and which accommodates a sensor element 10 between the nozzles 33 and the nozzle 35. The sensor element 10 is fixed to the inside of the housing 30 via elastic members 36 and 37 (e.g. o-rings).
FIGS. 6(a) and 6(b) show an example of the sensor element used in the above axial flow particle sensor. FIG. 6(a) is a plan view of the sensor element and FIG. 6(b) is a sectional view of the sensor element taken at the II--II line of FIG. 6(a). The vibrating section 14 is a thin plate and is fixed to a fixing section 16 so that the vibrating section 14 can vibrate, whereby the vibrating section 14 and the fixing section 16 forms a one-piece body 12. A hollow 17 is formed in the body 12 so that the vibrating section 14 can become a thin plate. On one surface 14s of the vibrating section 14 opposite to its surface facing the hollow 17 is provided a detecting section 20. In the portion of the body in the vicinity of the vibrating section 14 are formed a pair of throughholes 18 penetrating the body 12 in the thickness direction.
In FIG. 5, a fluid enters the housing 30 through the nozzle 33 and contacts with the vibrating section 14 and the detecting section 20 mounted on the vibrating section 14, in such a way that the fluid flow is interrupted by the contact as shown by the arrows. At this time, the solid particles present in the fluid collide with the vibrating section 14 and the detecting section 20, whereby the vibrating section 14 and the detecting section 20 vibrate. After contact with the vibrating section 14 and the detecting section 20, the fluid passes through the throughholes 18, passes through the nozzle 35, and leaves the sensor. With this particle sensor, solid particles present in a fluid collide with the detecting section 20 (having a piezoelectric film) of the sensor element or with the vibrating section 14 (mounting the detecting section 20 thereon) of the sensor element; thereby, the vibrating section 14 and the detecting section 20 are vibrated; the piezoelectric film 22 converts the vibration into electric signals; the electric signals are outputted by electrodes 24, 26 holding the piezoelectric film between them.
In detecting solid particles present in a fluid by the use of a particle sensor such as mentioned above, the fluid often contains bubbles besides the solid particles when the fluid is a liquid such as oil or the like. The vibrating section of the particle sensor is vibrated as well by the collision of the bubbles and the signals of this vibration are outputted as well from the detecting section. To accurately know the concentration of solid particles in fluid, it is necessary to distinguish the signals given by the vibration caused by bubbles from the signals given by the vibration caused by solid particles.
The vibration of the particle sensor caused by the collision of solid particles generally appears as the primary resonance vibration of the sensor element, such as shown by the signal waveform of FIG. 7. Meanwhile, the vibration of the particle sensor caused by collision of bubbles often appears as a long-period vibration having a frequency lower than the primary resonance frequency of the sensor element, such as shown by the signal waveform of FIG. 8.
However, collision of bubbles sometimes allows the vibrating section to cause vibrations containing the primary resonance vibration of the sensor element, such as shown by the signal waveform of FIG. 9. Such a vibration containing the primary resonance vibration of the sensor element, caused by bubbles is difficult to distinguish from a vibration caused by solid particles; this may result in taking of bubble-caused signals for particle-caused signals when a conventional sensor is used, and has been one reason for the lower accuracy of conventional sensors.